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Questions and Answers: 2024

Question: This question relates to hemlocks and hemlock wooly adelgid (HWA). On January 31 in 2022 we had a very short …

• Question

  This question relates to hemlocks and hemlock wooly adelgid (HWA). On January 31 in 2022 we had a very short cold snap with temperature going down to -7F. My understanding is that HWA cannot survive temperatures <-4F. Since then I have not seen HWA in my many outings in Lexington conservation land. Is it possible this could give the existing hemlocks a chance to survive? I've heard that one side effect of climate change is more temporary cold snaps like this.

  Answer

  Dear bkatzenberg, good morning to you. It is true that the hemlock woolly adelgid has a limited cold tolerance and is "controlled", so to speak, by low temperatures. Study shows that very low temperatures later in the winter are more effective at producing mortality than earlier in the winter because the cold hardiness of this insect declines as the season progresses. But, it is more likely that climate change will allow this species to move further north because they do survive temperatures below -4 F (it depends on the duration of the cold, not just the actual temperature). Best wishes.